1. Field of the Invention
The invention relates to a method for determining a distance between a frequency modulated continuous wave (FMCW) ranging device and a target.
2. Description of the Related Art
Conventional FMCW ranging devices may be sound or microwave-based and are used for performing distance or level measurements in industrial process control, factory automation or automotive applications.
Here, a transmitted signal is modulated to periodically sweep over a predetermined frequency range. A received signal, which comprises echo signal portions from the object of interest and other obstacles, is mixed down with the transmitted signal and the frequency difference signal obtained is analyzed by, e.g., a Fast Fourier Transform (FFT) to obtain a frequency spectrum in which the echoes appear as peaks. The echoes or peaks in the frequency spectrum (also referred to as echo profile) may be separate or may overlap. The most commonly applied technique to identify an echo of interest relies on the power spectrum density (PSD), usually calculated by FFT, to estimate the frequency of interest as the frequency corresponding to the largest component of the FFT-PSD.
The received signal, and thus the frequency-domain echo profile, often contains not only the desired useful frequency caused by reflection on the target, but also undesirable interference components at different frequencies, which may be caused by internal reflections in the electronics and on the antenna, or by external reflections, such as on the bottoms of containers, and on container struts. These interference components create a critical difficulty in determining the frequency of interest in their immediate vicinity. It is therefore necessary to attempt to suppress these interference components effectively.
DE 43 27 333 A1 describes a method for eliminating interference components in the frequency spectrum of an FMCW radar level device. The interference components are assumed to be at constant frequencies and independent on the level to be measured. A reference measurement is initially made in an empty container to record these interference components. In subsequent normal measurements, when the container contains a liquid, the recorded interference components are corrected with the aid of the measured intensity of a first interference component and subtracted from the respective frequency spectra obtained.
U.S. Pat. No. 6,810,342 B1 discloses a similar method where also a reference measurement is also made initially in absence of a target to record a spectrum of interference frequencies. In subsequent normal measurements, the down-mixed frequency difference signal is sampled at a predetermined number of sampling points that are equidistant in time. The interference frequencies of the recorded spectrum are used to determine, from the sampled frequency difference signal, complex amplitudes that approximate the proportion of the frequency spectrum of the sampled signals caused by the interference frequencies. The frequency spectrum caused by interference frequencies is then subtracted from the sampled frequency difference signal.
The conventional methods are based on the assumption that the interference frequencies have virtually constant, a priori known frequencies and do not vary with time which in practice is often not the case. In fact, the interference frequencies may vary with environmental factors, such as temperature, humidity and over time, because of aging of the ranging device, so that there is no adequate knowledge of the interference signals. Furthermore, the conventional methods require a special set-up for a reference measurement shot in the air or into an empty vessel.
In particular, when a target gets close or proximate to the ranging device or its antenna, the received signal is severely distorted in a much degraded error performance compared to that of further range targets. One reason for the degradation of performance is the existence of a huge interference factor in close in range, generally known as ring-down. The interferences emanate from complicated sources, which include, strong reflections from within the measuring system (RF output, waveguides, pressure separation, antenna), and dispersion within the system's transmission paths.
These interferences are known in their existence, but unknown in their properties. When a target gets close, the received signal contains a desired useful frequency that is close to interference frequencies. These signals are mixed and make it hard for any evaluation algorithm to determine the distance of the target with high accuracy. Therefore, the close-in range very often forms a blanking zone and the specified measuring range starts from a distance of, e.g., 1 m from the antenna of the ranging device, or the measurement tolerance within that region is increased.